Conventional image recording apparatus used in general printing, photographic printing, and copying use a light-emitting element such as laser diodes (LD) or light-emitting diodes (LED) as a light source for emitting a light beam for exposure of a recording carrier such as a photosensitive material or member. An optical modulator is used to carry picture information on the light beam. An optical deflector typically in the form of a rotary polygonal mirror deflects the light beam in a plane or in a linear direction to conduct digital primary scanning of the recording carrier. Auxiliary scanning is conducted to rotate or transfer the recording carrier as the primary scanning is continued, forming an image on the recording carrier. Thus, highly stable image signals of high quality are obtained by combining a laser with an optical modulator for modulating electrical signals indicative of picture information into optical signals and a scanner using a polygonal mirror.
The polygonal mirror ensures constant rotation at high rotation speed ranges, but experiences a speed variation at low rotation speed ranges. As the rotation speed lowers, the speed variation increases, inducing a jitter of a picture element position as will be later described in conjunction with FIG. 4.
To produce a stable image with laser scanning using a polygonal mirror, a high precision motor or a motor having an increased inertia must be used. The high precision motor is expensive, with the increased cost of a polygonal mirror assembly. The motor having an increased inertia has a certain limit above which the precision of rotation in low speed ranges cannot be improved, failing to achieve sufficient stabilization in low speed ranges.
For an assumed scanning time of 1.2 msec. as used in image recording at a density of 400 lines/inch for 10 seconds, for example, when it is desired to reduce the jitter caused by a variation in rotation of a polygonal mirror to within .+-.0.0025%, it is necessary to correct the scanning time within 30 nsec. (=1.2 msec..times.0.000025=3.times.10.sup.-5 msec.). Assume that there are 3,000 picture elements per line, in order to obtain a precision of 30 nsec./line by controlling a clock pulse per picture element of 400 nsec. (1.2 msec./3000 picture elements), the clock pulse per picture element of 400 nsec. must be corrected in a unit of 0.01 nsec. (30 nsec./3000 picture elements). The base clock used for control in 0.01 nsec. unit must have a frequency of 100 GHz (=1/0.01.times.10.sup.-9 =100.times.10.sup.9). This frequency is impractical because it is not available with commercial circuit elements currently used in business machines.